title: A comparison of the gamma index analysis in various commercial IMRT/VMAT QA systems.
abstract: OBJECTIVE
  To investigate the variability of the global gamma index (γ) analysis in various commercial IMRT/VMAT QA systems and to assess the impact of measurement with low resolution detector arrays on γ.
  
  
  METHODS
  Five commercial QA systems (PTW 2D-Array, Scandidos Delta4, SunNuclear ArcCHECK, Varian EPID, and Gafchromic EBT2 film) were investigated. The response of γ analysis to deliberately introduced errors in pelvis and head & neck IMRT and RapidArc™ plans was evaluated in each system. A theoretical γ was calculated in each commercial QA system software (PTW Verisoft, Delta4 software, SNC Patient, Varian Portal Dosimetry and IBA OmniPro, respectively), using treatment planning system resolution virtual measurements and compared to an independent calculation. Error-induced plans were measured on a linear accelerator and were evaluated against the error-free dose distribution calculated using Varian Eclipse™ in the relevant phantom CT scan. In all cases, global γ was used with a 20% threshold relative to a point selected in a high dose and low gradient region. The γ based on measurement was compared against the theoretical to evaluate the response of each system.
  
  
  RESULTS
  There was statistically good agreement between the predicted γ based on the virtual measurements from each software (concordance correlation coefficient, ρc>0.92) relative to the independent prediction in all cases. For the actual measured data, the agreement with the predicted γ reduces with tightening passing criteria and the variability between the different systems increases. This indicates that the detector array configuration and resolution have greater impact on the experimental calculation of γ due to under-sampling of the dose distribution, blurring effects, noise, or a combination.
  
  
  CONCLUSIONS
  It is important to understand the response and limitations of the gamma index analysis combined with the equipment in use. For the same pass-rate criteria, different devices and software combinations exhibit varying levels of agreement with the predicted γ analysis.
authors.affiliation: Department of Medical Physics, Royal Surrey County Hospital, Guildford, UK; Centre for Nuclear and Radiation Physics, University of Surrey, UK. Electronic address: m.hussein@nhs.net.
authors.forename: Mohammad
authors.initials: M
authors.lastname: Hussein
authors.forename: Pejman
authors.initials: P
authors.lastname: Rowshanfarzad
authors.forename: Martin A
authors.initials: MA
authors.lastname: Ebert
authors.forename: Andrew
authors.initials: A
authors.lastname: Nisbet
authors.forename: Catharine H
authors.initials: CH
authors.lastname: Clark
mesh_terms: D005720:Gamma Rays; D006801:Humans; D009369:Neoplasms / Q000532:radiotherapy; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011785:Quality Assurance, Health Care; D011874:Radiometry / Q000295:instrumentation / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D012984:Software; D014057:Tomography, X-Ray Computed / Q000295:instrumentation / Q000379:methods
keywords: Detector arrays; Gamma index; IMRT; VMAT